Engine Setback

[Doug1]

Any good links to calculating the advantages of moving the engine back and lower in a car. I'd like actual formulas if possible.

[Rod]

.?..? That varys from car to car, engines original location, car weight, engine weight, engine accs(cast iron heads, intakes, ac, pulleys) you might need to find a completed car like youres corner scale it, build the offset car and repete...on my own car I used the short motor mounts and 1 inch back motor mounts to help the 50-50 balance of my car ending up with 53-47 without any structural changes

[19,69camaro]

Don't forget what happens when you move the driver back a few inches too.

[Kenny]

No good links on calculating advantages in a raw form as far as I know. The amount weight distribution is changed is basic math, but consideration needs to be given to the fact that you are moving a % of weight a % rearward. It is not a very effective way to improve weight distribution when you consider the mods required to make a big difference, unless you really like sitting directly beside your engine. If you have a 3500 lb car and you move a 35 lb battery 100% to the opposite end, you will see a 1% distribution change. That would be from the outer extreme of one end to the extreme opposite end. The problem with the engine is that it is farther back and already supported in some percentage by the rear so a large amount of weight is moved a very small distance in comparison. If you can move the engine back a few inches, by all means do it, unless it is gonna create a bunch of expense or make service too difficult.

https://www.pro-touring.com/showthre...fair+advantage

[Motorcitydak]

Engine set back, I have some of that. My set back was more dictated by the amount of space available vs perfect distribution however



I do agree that a 50/50 weight distribution is seeming ideal for any car but that would only be if the traction available at all 4 corners is the same with same tire size. A typical car has wider tires in the rear than the front so there is more traction available there. If you have more weight on the back it may actually help. What it would come down to if the pounds per square inch of contact between the tire and the ground. If you have ever seen an F1 car being lifted in the air by a single central point, you will see that the rear end of those cars hangs down a lot more than the front. They do not appear to have a 50/50 weight split either, more weight in the rear and their rear tires are larger than the fronts

[exwestracer]

Starting to feel like a thread hog here, but you guys are bringing up some really interesting stuff lately!

Doug, first I would define "advantage". You asked a VERY broad question... What is it you are trying to achieve? Quick turn in? Neutral handling at the traction limit? Controllable oversteer? If you are designing a chassis from scratch, each of those would dictate a very different F/R weight distribution. I am a firm believer in low polar moment of inertia; that is, centering most of the vehicle mass between the axle centerlines. Setback is obviously an easy way to achieve this.

As Kenny pointed out above, the math is pretty simple; IF you have accurate weights for the total vehicle and engine/trans combo. Let's say we have a 3000lb total weight, and a 600lb engine/trans package that we are moving around. We'll use a 100" wheelbase, just to keep the math simple. So every inch we move the engine is 1% of the wheelbase. The engine package is 20% of the total weight.

I'm going to make 1 more assumption...let's say that if we took the engine and trans OUT and weighed the car we would have 50/50 distribution. 1200lb on the front axle, and 1200lb on the rear axle. If we start out with the engine center of mass at the front axle centerline (not realistic, but bear with me), we would have 1800 front /1200 rear, or 60/40 front distribution. The MOST we could achieve is a 20% change if we moved it all the way to the rear axle C/L... 1200 front /1800 rear. Still with me? Now, let's take that 20% swing and divide it by the wheelbase, 100". That means every inch we move the engine back changes the F/R distribution 0.2%.

Again, you have to KNOW the exact weights and distances you are dealing with. Making a wrong "guess" at any of it makes the calculation worthless...

[Ron.in.SoCal]

Ray, please spare me the 'thread hog' comments. I look forward to your every post and thank you for your past feedback via PM. On to the topic...

So if I'm reading you correctly, you' re saying above every inch is .2% and 2 inches are basically less than a .5% movement in f/r distribution (i.e. next to nothing). Doesn't seem worth much unless you're moving 10-12 inches.

[wellis77]

Engine set back, I have some of that. My set back was more dictated by the amount of space available vs perfect distribution however

Ditto that. I'd like to be as close to "balanced" as possible but the reality is, I had to in order to set mine back to fit this pig in the engine bay.

[exwestracer]

Sounds like a waste of time. doesn't it? Well, if we look at it in pounds instead of percentage, every inch we set the engine back is adding 6 pounds to the rear and taking it off the front, right?

Doug also mentioned engine height, and I'd like to address that as well. In one of my rainy day threads, I discussed roll moment length. This is the leverage distance the C/G height has over the roll center. In short, it has everything to do with how much the car wants to roll over entering a corner. Now we are dealing with a much shorter distance, say 12-18" for a typical P/T type car. Dropping the engine and trans 2" makes a HUGE difference in chassis roll, which allows us to run LESS BAR. A good quality (light) carbon fiber hood will help this out a bunch as well. Obviously, ground clearance is a major practical issue, but anything we can do to get the weight down will be an improvement.

My kid has a bone stock 2011 Scion xB (the box) that he took to a practice day autocross once. Stop laughing, he set the best time for a street tire car that day... Anyway, one of the things I had him try was folding all the seats down between runs. Yup, you guessed it...the car cornered flatter and went faster than it did with them up.

[Kenny]

That is also a very good point. Smaller bars will equal more consistent behavior over various surfaces and ease the compromises in shock valving.

F/R percentages are also misleading as a raw number without some idea of concentration of mass. The famous Herb Adams statement that a bowling ball changes direction easier than a dumbbell is true. The more weight you can move from outside the wheelbase the better the chassis dynamics. The same thing is true when considering how weight is distributed around the CG. More centralized mass is better even if the percentage numbers are slightly skewed . I started tooling around with manipulating "cantilevered" weight to manipulate balance..... Turns out that weight that is centrally located is FAR easier to control in a dynamic state, even if it is not as "balanced" by static measurement. Picture bicycle wheel vs the aforementioned bowling ball. Engine setback is worthwhile, just not a deal breaker if you can't do it.

You should make a concentrated effort on relocating/removing weight that is located outside the wheelbase, and moving components lower in the car.

[Rod]

Thread hog!!!! Ha ha ha ba haha

[ace_xp2]

I'd always thought that going beyond 50% rear was also to help with initial turn in. As the front tires are not only turning in but trying to convince the back tires to do the same thing, so a rearward weight balance will help before the springs can.

[Norm Peterson]

So if I'm reading you correctly, you' re saying above every inch is .2% and 2 inches are basically less than a .5% movement in f/r distribution (i.e. next to nothing). Doesn't seem worth much unless you're moving 10-12 inches.

What he's really saying is that you need to know the actual weights, where their individual CG's are originally located, and where they are hopefully ending up.

The rest is simple beam formulas out of any structural engineering textbook (relax, the ones you'd be using aren't at all complicated).

It's actually a useful little problem to throw at an Excel spreadsheet, if you're so inclined.


Norm

[Norm Peterson]

I'd always thought that going beyond 50% rear was also to help with initial turn in. As the front tires are not only turning in but trying to convince the back tires to do the same thing, so a rearward weight balance will help before the springs can.

The "helping with initial turn-in" part may be more a consequence of a rear-biased weight distribution tending to have a lower polar moment of inertia than 50/50 or more nose-heavy. Pretty much by definition, the engine, clutch or torque converter, and transmission have to have been moved back closer to the other heavy bits (axle, fuel tank), and might have dragged the radiator and its support structure back with them as well.


Norm

[MonzaRacer]

So basic information first,car has a balance point front to back. This is if you lifted car on floor jack, with a triangular wedge on it, at this point car would "balance like a teeter totter" this is "center" of car.
Now from center ,front of camshaft draw a line till it intersects with this point (that pivot point should be plotted straight up and down and parallel to road surface.) .
Then all of your pivot points in rear suspension plot forward also.
The plots forward will intersect in most cases and give you a "lift point" that is where the rear suspension lifts from. Higher up in car its lifting MORE of the car, lower in car (up and down its having to push it UP, as if the load is above your hand loading back of hand or your pulling it like with a string.
Now your can change how this loads rear suspension and tires by how far forward or rearwards and higher or lower.
Lots of measuring, plotting, and such.
Fred Puhn has a book "How to Make Your Car Handle" that explains so much more. AND it also depends on the rears suspension mounting points designed into a specific chassis (http://www.amazon.com/How-Make-Your-...our+Car+Handle)
Setting an engine back and lower pushes the lifting ability of the rear suspension farther. Basically it takes weight OFF the front of car, centralizes the heavy spot of current car technology (ever see where the engine is in a Detomaso Pantera? ) closer to middle of car, and also adds weight to rear of car to load the rear suspension more,,,but that was when gassers had engine high and set back, road racers want weight lower in car and farther back. It also changes the loading of front suspension,especially in turns and corners, trying to even out corner weights. Ever watch tractor pulls or truck pulls, reason they try to move weight around is so tractor "gets up" on rear wheels for maximum traction loading of rear tires. same goes for drag racing.
Most circle track cars have a no setback rule or not. cheating here makes cars handle better.
The lower part id in center of gravity, as in how heavy the car is in a turn, less weight up high lets you run less spring rate and different shock damping.
The rule of thumb Iwas taught and learned the LESS you have to control the more that it takes care of its self. IE less things to think about.
honestly unless your road racing or competing for huge amounts of cash, proper suspension prep and just learning HOW to tune what you have rather than monkey with lots of physics dynamics that get you overly technical in building a car.
But lets say you bought a Chevelle with bad frame and floor, build your own frame, move engine back, make it a two seater and take advantage of benefits. But it also, in trying to lower engines mandated developing dry sump systems, so your oil pump isnt down low, your engine is. and hence rules to LIMIT set back and lowering.
Get the book, its good read and explains its MUCH better than I ever could on here, Oh and the pics of the top fuel cars launching REALLY show off TRACTION at its best.
Good Luck
Lee